The invention relates to an electro-hydraulic actuator for brake system or the like, of the type having a fluid filled working chamber.
Numerous types of known electro-hydraulic actuators for converting electrical energy into hydromechanical energy are used, for example as the master or wheel brake cylinders of vehicle braking systems, for periodic pressure relief in antilock brake systems, as electrically powered liquid pumps, or in the form of electro-hydraulic positioning drives. A common feature of these actuators is that the electrical input energy is initially converted into purely mechanical working energy, which is then converted into a corresponding change in pressure or volume in the fluid-filled working chamber of the actuator. As a result, such actuators usually exhibit undesirably high power losses and corresponding low efficiency, are limited to low-frequency regulating speeds, and/or require relatively costly wear-prone design.
German patent document DE-OS 41 27 860 discloses a pump system for conveying liquid or gaseous media, in which a piezoelectric coated film is located in a working chamber that is filled with a constant hydraulic volume and is separated from the feed chamber of the pump by a diaphragm. The film is periodically charged with an electrical voltage to produce a periodic increase in the volume of the working chamber as a result of a change in thickness of the piezoelectric film, thereby transporting liquid or gas by a pulsating movement. A pump system of this kind is superior to a conventional design in many respects, but has the disadvantage that the changes in volume that can be achieved, based on the total volume of the working chamber, are extremely small, first because a piezoelectric, when undergoing an increase in length, also undergoes a transverse contraction in the opposite direction under the influence of an electrical voltage, and therefore naturally has a very low volumetric expansion coefficient; and second because the working chamber can be filled only partially with the piezoelectric film while the remainder of the working chamber volume is required for the hydraulic filling that does not participate in the electrically induced volume change.
The goal of the present invention is to provide an electro-hydraulic actuator of the species recited at the outset which has low-inertia, high-frequency adjustability and a very simple, low-wear, space-saving design.
This goal is achieved by the electro-hydraulic actuator according to the invention in which the working chamber of the actuator contains an electrostrictive fluid, and an electrically controlled electrode arrangement is provided to generate an electric field which acts on the fluid. By way of the positive or negative expansion behavior of the electrostrictive fluid, input electrical energy is thus converted directly, without interposition of any kind of mechanically moved components, into output energy of the actuator that can be used to change the volume and/or pressure. Therefore, in comparison to the piezoelectric pump system described at the outset, significantly higher changes in volume based on the total volume of the working chamber can be achieved, first because the working chamber is filled completely with the electrostrictive medium, and second because electrostrictive fluids do not exhibit a transverse contraction, and can achieve a much higher volumetric expansion coefficient than piezoelectrics. The actuator according to the invention has an extremely simple, problem-free, low-loss, yet space-saving design and has the ability to be regulated very quickly, allowing working frequencies of more than 1 kHz.
The effective pressure level or working volume of the actuator according to the invention can be regulated solely by the voltage potential applied to the electrode system, to any value within fixed limits, so that for this purpose the volume of fluid contained in the working chamber does not have to be changed. Thus, in another structurally advantageous embodiment of the invention, the working chamber has a completely enclosed design and is filled with a constant amount of electrostrictive fluid.
The changes in volume of the actuator produced by the electrostriction effects are advantageously transmitted to the exterior by a movable limiting wall of the working chamber, which preferably consists of a flexible diaphragm. For use of the actuator as a positioning drive or for valve actuation, the movable limiting wall of the working chamber is coupled with the positioning element of the positioning drive. The positioning path of the positioning element is therefore adjustable depending on the voltage level applied to the electrode system.
Optionally or additionally, the pressure and/or volume changes in the electrostrictive fluid are also transmitted in a structurally simple fashion, to a fluid (preferably hydraulic) system separated from the working chamber. A hydraulic chamber abutting the working chamber of the actuator is selectively pressurized and relieved of pressure by the electrostriction effect produced in the working chamber by means of a movable partition. The hydraulic chamber separated from the electrostrictive fluid is preferably the delivery chamber of a liquid pump.
According to another especially advantageous embodiment of the invention, the actuator is used for selective pressure regulation of a hydraulic brake system for lowering brake pressure in the event of a threatened lockup of the vehicle wheels in an antilock braking system.
The electrode system in the working chamber is advantageously made from a plurality of individual electrodes, running parallel to one another and connected to have alternately positive and negative polarity, in order to use a relatively low operating voltage to generate a sufficiently strong electrical field that traverses the working chamber essentially uniformly.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.